1. Field of the Invention
The present invention relates to a liquid crystal display and a method for manufacturing the liquid crystal display and, more particularly, to a liquid crystal display with a liability electrical connection from external input terminals on one substrate to electrodes on another substrate and a method for manufacturing the liquid crystal display.
2. Description of the Related Art
Active matrix liquid crystal displays ordinarily have a structure in which a TFT substrate and a counter substrate opposed to the TFT substrate are bonded to each other by a sealing material and a liquid crystal material is enclosed between the TFT substrate and the counter substrate. In order to apply a drive voltage to this liquid crystal material, pixel electrodes and an input terminal portion including input terminals electrically connected to the pixel electrodes are formed on the TFT substrate. A counter electrode (common electrode) is formed on the counter substrate at a position such as to face the pixel electrodes on the TFT substrate. Conventionally, by providing counter electrode terminals in the input terminal portion of the TFT substrate an electrical connection between the counter electrode terminals and the counter electrode on the counter substrate is established.
As a first conventional technique, Japanese published application H5-127172A discloses a technique for establishing electrical connection between the counter electrode terminals in the input terminal portion of the TFT substrate and the counter electrode on the counter substrate. FIGS. 1 (a) and (b) show the technique in which conductive columns formed mainly of a conductive paste are formed so as to be surrounded by a seal for sealing the two substrates. FIG. 1(a) is a plan view of a liquid crystal display according to this conventional technique, and FIG. 1(b) is a cross-sectional view. Electrodes 103 and 106 are respectively formed on a TFT substrate 102 and a counter substrate 104, and conductive columns 120 formed mainly of a conductive paste are formed between the electrodes 103 and the electrodes 106. The electrodes 103 are counter electrode terminals provided in an input terminal portion 107 of the TFT substrate 102. The electrodes 106 are connected to a counter electrode 106′ on the counter substrate 104. The counter electrode terminals in the input terminal portion 107 of the TFT substrate 102 and the counter electrode 106′ on the substrate 104 are thereby connected electrically to each other. Further, the conductive columns 120 are surrounded by a seal 105 for bonding together the TFT substrate 102 and the counter substrate 104.
FIG. 2 is a flowchart of for explaining an example of a process for manufacturing this conventional liquid crystal display. First, an alignment film is formed on the TFT substrate 102 on which electrodes 103 and pixel electrodes 124 are formed and the electrodes 106 and the counter electrode 106′ are formed on the counter substrate 104 (steps S101′ and S101), followed by rubbing (steps S102′ and S102) and cleansing after rubbing (steps S103′ and S103). Subsequently, silver paste for forming conductive columns 120 is applied to the electrodes 103 on the TFT substrate 102 (step S104′). On the other hand, sealing material 105 is applied to portions of the substrate 104 other than the portions on which the electrodes 106 and the counter electrode 106′ are formed (step S104). Thereafter, the two substrates 102 and 104 are bonded together (step S105), a liquid crystal is injected (step S106), and the liquid crystal injection hole is closed (step S107). The process of manufacturing the liquid crystal display according to this conventional technique is thus performed.
As a second conventional technique, Japanese published application 2001-5017A discloses a conventional technique for establishing electrical connection between counter electrode terminals and a counter electrode. FIG. 3 shows the technique using conductive columns formed by using color filter materials laminated on the periphery of a display region. FIG. 3 is a cross-sectional view of a portion on the periphery of a display region of a liquid crystal display according to this conventional technique. Columns are formed on a counter substrate 204 by laminating a plurality of color filter materials 214. A counter electrode 206 formed on the counter substrate 204 so as to face pixel electrodes 224 on a TFT substrate 202 extends to the columns and covers the surfaces of the columns. The columns and counter electrode 206 formed on the columns form conductive columns 220. Electrodes 203 are formed on the TFT substrate 202 so as to contact the bottom surfaces of the conductive columns 220. Counter electrode terminals (not shown) connected to the electrodes 203 and the counter electrode 206 are thereby connected electrically to each other. A seal 205 is formed outside the conductive columns 220.
As a third conventional technique, Japanese Patent No. 3014291 discloses a conventional technique for establishing electrical connection between counter electrode terminals and a counter electrode. The technique uses conductive columns formed by using color filter materials laminated in a display region of a liquid crystal display. FIG. 4 is a cross-sectional view of a portion on the periphery of a display region of a liquid crystal display according to this conventional technique. Columns are formed on a counter substrate 304 by laminating a plurality of color filter materials 314. A counter electrode 306 is formed so as to cover the columns. The columns and the portions of the counter electrode covering the columns form conductive columns 320. The conductive columns 320 are in contact with accumulating capacitor lines 325 on a TFT substrate 302. The accumulating capacitor lines 325 are connected to counter electrode terminals (not shown) to establish electrical connection between the counter electrode terminals and the counter electrode 306.
However, in the first conventional technique, since a material having silver paste as a main constituent is used for the conductive columns 120, one step of applying silver paste to one of the TFT substrate and the counter substrate is required for electrical connection between the TFT substrate and the counter substrate, as shown in the flowchart of FIG. 2. Application of silver paste is performed by dotting silver paste on the substrate by a dispenser method. In this method, since the amount of silver paste applied from a dispenser varies, it is necessary to increase the amount of applied paste in order to stabilize the electrical connection and, therefore, there is a problem that the application area is increased. Further, in the case of multiple patterning for forming a plurality of small liquid crystal displays on a large substrate, the number of silver paste dots is increased in proportion to the number of liquid crystal displays formed on the substrate and there is a problem that the process time is increased and the throughput is reduced. Further, there is also a problem that a foreign material may attach to the silver paste or that silver paste may be sputtered to portions other than the application area by the operation of the dispenser to cause a display fault. Further because of large variations in application pressure the amount of application can not be uniformly maintained and the stability of electrical connection between the TFT substrate and the counter substrate is reduced. There is also a possibility of the stability of electrical connection between the electrodes 103 and 106 being reduced due to the difference between the amounts of shrinkage of the sealing material and silver paste in the bonding step.
In the second conventional technique, the conductive columns 220 and the electrodes 203 on the TFT substrate 202 are electrically connected only by the pressure applied by means of the seal 205 formed outside the conductive columns 220 to constantly maintain the spacing between the TFT substrate 202 and the counter substrate 204. Therefore, there is a problem that adhesion between the conductive columns 220 and the electrodes 203 on the TFT substrate 202 is so low that the electrical connection is instable. This tendency has become stronger in recent years particularly with the further reduction in thickness of glass substrates used as TFT and counter substrates, because the substrates can be distorted or warped, for example, by an environmental change such as a change in temperature. In extreme cases, the contact portions are disconnected or separated from each other.
In the third conventional technique, the structure on the periphery of the conductive columns 320 in high-precision liquid crystal displays having smaller pixels and a smaller pixel spacing is complicated, resulting in an increase in the number of steps in the manufacturing process and an increase in the manufacturing cost. For establishment of electrical connection between the accumulating capacitor lines 325 and conductive columns 320, troublesome operations are required such as forming holes in gate insulating film 326 on the accumulating capacitor lines 325 and shaving off through the holes the alignment film (not shown) on the accumulating capacitor lines 325 at the bottoms of the conductive columns in order to establish connection between the accumulating capacitor lines 325 and the conductive columns 320. In addition, the conduction between the accumulating capacitor lines 325 and the conductive columns 320 is instable. Further, since the conductive columns 320 on which the counter electrode 306 are formed exist in the display region, rubbing is difficult to perform and the alignment of the liquid crystal material in the vicinity of the conductive columns 320 is disturbed, resulting in a considerable reduction in display quality.